Derived set is the basic concept of set theory and topology. Let a be a subset of topological space (x, τ). The set of all the aggregation points of a is called the derivative set of a, and it is recorded as A'. Expressed in mathematical language is: A'={x∈X| For any open domain U(x)∈τ, there must be y≠x, so that y∈U(x)∩A}. Features: Exclude isolated points